Molecular cloning studies have demonstrated that receptors for steroids, retinoids, vitamin D and thyroid hormones comprise a superfamily of regulatory proteins that are structurally and functionally related (see Evans, in Science 240:889–895 (1988)). Known as nuclear receptors, these proteins bind to cis-acting elements in the promoters of their target genes and modulate gene expression in response to ligand therefor, such as a hormone.
Nuclear receptors can be classified based on their DNA binding properties (see Evans, supra and Glass, in Endocr. Rev. 15:391–407 (1994)). For example, the glucocorticoid, estrogen, androgen, progestin and mineralocorticoid receptors bind as homodimers to hormone response elements (HREs) organized as inverted repeats (IRs, see Glass, supra). A second class of receptors, including those activated by retinoic acid, thyroid hormone, vitamin D3, fatty acids/peroxisome proliferators and ecdysone, bind to HREs as heterodimers with a common partner, the retinoid X receptor (i.e., RXR, also known as the 9-cis retinoic acid receptor; see, for example, Levin et al., in Nature 355:359–361 (1992) and Heyman et al., in Cell 68:397–406 (1992)).
An important advance in the characterization of the nuclear receptor superfamily of regulatory proteins has been the delineation of a growing number of gene products which possess the structural features of nuclear receptors, but which lack known ligands. Accordingly, such receptors are referred to as orphan receptors. The search for activators for orphan receptors has created exciting areas of research on previously unknown signaling pathways (see, for example, Levin et al., (1992), supra and Heyman et al., (1992), supra). Indeed, the ability to identify novel regulatory systems has important implications in physiology as well as human disease and methods for the treatment thereof.
Since receptors have been identified for all known nuclear-acting hormones, a question arises as to the types of molecules that may activate orphan receptors. In view of the fact that products of intermediary metabolism act as transcriptional regulators in bacteria and yeast, such molecules may serve similar functions in higher organisms (see, for example, Tomkins, in Science 189:760–763 (1975) and O'Malley, in Endocrinology 125:1119–1120 (1989)). For example, a crucial biosynthetic pathway in higher eucaryotes is the mevalonate pathway (see FIG. 1) which leads to the synthesis of cholesterol, bile acids, porphyrin, dolichol, ubiquinone, carotenoids, retinoids, vitamin D, steroid hormones and farnesylated proteins.
Farnesyl pyrophosphate (FPP), the metabolically active form of farnesol, represents the last precursor common to all branches of the mevalonate pathway (see FIG. 1). As a result, FPP is required for such fundamental biological processes as membrane biosynthesis, hormonal regulation, lipid absorption, glycoprotein synthesis, electron transport and cell growth (see Goldstein and Brown, in Nature 343:425–430 (1990)). Because of the central role of FPP in the production of numerous biologically important compounds, it is to be expected that its concentration should be closely regulated. This suggests that cells are likely to have developed strategies to sense and respond to changing levels of FPP, or its metabolites. One possible strategy by which cells can accomplish the desired regulation is to utilize a transcription factor whose activity is specifically regulated by a low molecular weight signaling molecule such as an FPP-like molecule. Potential candidates for such means to sense changing levels of FPP, or metabolites thereof, include members of the nuclear receptor superfamily, since these proteins are activated by low molecular weight signaling molecules.